10 Plants and Chemicals Toxic to Horses

While most equestrians and horse owners know about the common plants toxic to horses, sometimes they are unaware that other toxic plants and chemicals are located on their horse property. Keep reading to learn what to watch out for!

1. Yew

American, English, Japanese, and Western yew are ornamental evergreen hedge-type plants that grow red berries in the fall. They are commonly used in landscaping across much of North America.

Danger to Horses  As little as a mouthful or two of yew can be lethal.The plant’s alkaloid toxin taxine causes cardiac and respiratory failure which usually happens within minutes. 

Risk of Exposure  Most yew poisonings occur when clippings are erroneously tossed into a pasture after trimming, with leaves remaining toxic even after they wilt. Decorative wreaths made with yew are another potential source of exposure when hung where horses can access them.

2. Oleander

This common decorative perennial evergreen shrub produces white, pink, or red flowers in spring and summer. Common in the southern United States, oleander grows only in climates where temperatures remain above freezing.

Danger to Horses  Potent cardiac glycosides in the plant affect the heart’s ion balance, causing irregular heart activity that can ultimately result in cardiac failure and death. Relatively small quantities (0.005% of the horse’s body weight, or 0.05 pounds for a 1,000-pound horse) are considered lethal. Ingestion might also cause colic.

Risk of Exposure  Horses are often exposed to oleander when people toss clippings into pastures. Horses ridden on trails might also take a quick bite out of oleander.

3. Ionophores

These antibiotic feed additives, such as monensin, are used as growth promoters in cattle and poultry diets. Farmers also use these as antiprotozoal agents to control Coccidial Infections.

Danger to Horses  Horses are more sensitive than other livestock to ionophores, which influence ion transfer across cell membranes and, thus, affect how nerves and muscles function. Clinical signs of consumption can include a loss of appetite, rapid heart rate, sweating, colic, and unexpected death. 

Risk of Exposure  Horse feed can become contaminated with ionophores if manufacturers producing a variety of feed types don’t follow proper cleaning protocols between formulating batches for different species. 

4. Blister Beetles

Abundant in the Midwestern United States, blister beetles swarm alfalfa fields and can be baled into alfalfa hay during harvest. Present from mid to late summer, they feed on the tops of alfalfa plants.

Danger to Horses  Blister beetles contain cantharidin, a toxic chemical and blistering agent. Clinical signs typically appear within hours of ingestion and include gastrointestinal distress, strained and frequent urination, and lesions in and around the mouth. If a horse ingests a lethal quantity (believed to be 0.5-1 mg of cantharidin per kilogram of body weight), he can die within 72 hours.

Risk of Exposure  Alfalfa hay can become contaminated with beetles that are crushed during the crimping process (when hay stems are broken to hasten drying). Even dried and crushed, the beetles remain toxic for a long time. Only a few flakes of hay in a bale might be affected because of the beetles’ tendancy to swarm. Unfortunately, short of carefully inspecting each flake, there is no effective way to assess for contamination. It is recommended to buy hay from farmers that take precaution when harvesting the hay

5. Rodenticides and Pesticides

These are products formulated to kill rats and mice, gophers, birds, snails, slugs, ants, or other offending pests. They often consist of a pelleted, granular, or powdered bait.

Danger to Horses  In sufficient doses the toxic agents used in rodenticides and pesticides can be lethal to horses. Anticoagulant rodenticides, for example, a common class of rat poison, are designed to cause bleeding and hemorrhaging in the targeted pest but ultimately can cause the same in larger species. 

Risk of Exposure  Bait-type products often contain sweet flavorings or grain bases designed to attract pests. These qualities often make them attractive to horses as well. Exposure typically occurs due to inappropriate storage or because those setting the bait do not follow label directions.

6. Herbicides

Some property owners apply herbicides to their fields to control weeds. Glyphosate and phenoxy herbicides are most commonly used.

Danger to Horses  Horses might be inclined to consume toxic plants they normally wouldn’t eat after they have been sprayed with herbicide. Diarrhea and colic are typically seen post-exposure.

Risk of Exposure  The most common means of exposure is when an owner has used an herbicide in a horse’s field and forgotten about it. 

7. Decaying Organic Matter

Rotting hay, haylage, or other organic matter has the potential to harbor botulism-causing toxins produced by Clostridium botulinum.

Danger to Horses  Horses are highly susceptible to C. botulinum toxins, which attack the nerves that communicate with muscles, leading to general weakness that progresses to paralysis. Clinical signs of botulism might include inability to eat, drooling, nasal discharge, muscle tremors, difficulty getting up, difficulty breathing, and death.

Risk of Exposure  Toxin ingestion is one of the most common routes of exposure in horses, though it can also be acquired through exposed wounds. Some parts of the country, such as Kentucky, have high levels of the toxin-producing bacterial spores in the soil. 

8. Fumonisin (moldy corn)

This mycotoxin (fungal toxin) can infect corn prior to harvest or during storage. Hot, dry conditions followed by high humidity are associated with increased fumonisin concentrations in growing corn, usually in the Midwest and South.

Danger to Horses  Horses that eat corn containing toxic fumonisin levels develop moldy corn poisoning, or equine leukoencephalomalacia (ELEM), a rapidly progressing, often fatal neurologic disease. Clinical signs might include those typical of neurologic disease, such as head-pressing, circling, muscle tremors, weakness, or strange, sometimes violent behavior.

Risk of exposure  Most commercial feed mills test for fumonisin contamination. But feeding untested corn, such as that which has come directly from the field, can put horses at risk, particularly in regions where fumonisin is more prevalent.

9. Red maple

Researchers have known that wilted red maple leaves can be toxic to horses, but they now suggest that other species, such as sugar and silver maple, might be problematic as well.

Danger to Horses  While research on the mechanism behind red maple poisoning is ongoing, scientists believe the toxic agent is linked to levels of gallic acid, which increase in leaves throughout the summer. The leaves in combination with certain bacteria produce a strong oxidant that damages horses’ red blood cells, hindering their ability to carry oxygen or destroying them completely. Clinical signs include loss of appetite, red urine, increased drinking and urination, and a generally depressed state. To be affected, an average-sized horse would need to consume an estimated 1.5-3.3 pounds of wilted leaves.

Risk of Exposure  Red maple poisoning happens mostly in the late summer and early fall. Fallen branches in a paddock following a storm are the most common source of exposure, with wilted leaves remaining toxic for as long as 30 days. Bored or curious, a horse might strip the leaves off and eat them. Scientists believe the bark is also toxic.

10. Tansy ragwort

A non distinct yellow flowering plant, tansy ragwort grows throughout most of North America.

Danger to Horses  If a horse eats enough of the plant over a short period of time, or smaller amounts over a longer period of time, they can develop an irreversible chronic liver disease, though they may not show signs for six months to a year. Signs can be neurologic and include head-pressing, circling, and bizarre behavior. Loss of appetite and weight loss over time are also common.

Risk of Exposure  Horses might consume the plant if it gets baled into hay, but because disease onset is slow and clinical signs often do not appear until well after the initial exposure, poisoning can be difficult to trace.

Carolina Equine Hospital

Do you suspect your horse has gotten into something toxic? Give us a call! It is also handy to have the number for your local agricultural agent in your emergency contact list!